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Gain is defined as the unit of measurement used to mean the ratio of a signal output of a system to the signal input of the system. Gain is a term most commonly associated with amplifiers and amplification. As you capture sound the mic, camera and the environment generate a certain amount of noise. If the level of the audio capture is low, most times the noise ends up being more prominent so you signal to noise ratio is lower than it could be. So when recording your video in order to produce the least noisy recording the levels should be seat as high as possible with being too high causing clipping or serious distortion of the
signal.

When putting your video together and editing you need to keep in mind the audio may have come from a variety of sources and or areas which all will have a direct impact on levels and in some cases causing a great variation in the levels. (For example, if you are recording the audio in a room and when you begin recording its just you. Then during the recording someone enters the room with a sweater on (sound absorbing material). The type of clothing they are wearing can actually affect the levels being recorded.) Using the Gain Filter allows you to amplify or attenuate the levels of your audio clips much greater than the default gain available associated to the audio level key frames. As you did by adding the Gain filter you attempted to amplify the audio levels of your clips. This is fine however it has associated risks. One risk is similar to what you describe and
similar to what I hear in the video you posted - manually adjusting the gain
levels of a clip too high or past the peak audio levels you recorded will result in distorted audio. With that said, there are a few things you can do to your audio clip before adding the Gain Filter. To help ensure your clips with low audio levels have the optimal gain, normalize the audio clips you are working with. The way normalization works is the audio clip is scanned for the loudest sample level and then applying a Gain filter that adjusted that loudest (peak) level to an acceptable level with no distortion. This level by default is 0 dBFS which is the highest possible level before the audio is clipped (clipping is the distortion you hear). Once the clip is normalized and the peaks are corrected the Gain filter can be used to raise the overall audio level. Something to keep in mind is that normalization is at least in most NLE's a destructive process in which audio clips/files are permanently modified.

So in your case you state that the mic delivers decent quality which might be true for the levels you had set but the signal-to-noise ratio might be wrong. At lower audio levels the audio might sound fine however by adding the Gain filter you may be amplifying the noise due to low audio level or introducing clipping (which
sounds like what is happening). There is a great deal that goes into great sound/audio. It really all comes down to ensuring you get it right in the beginning by setting your initial audio levels correctly. After listening to the video you posted I would start with normalization but first if you added any filters remove them and if you used any noise reduction remove it also.